Response of irrigated soybeans (Glycine max) to phosphorus on an alkaline cracking clay in semi-arid tropical Australia

Abstract

The primary and residual effects of phosphorus (P) fertilizer on soybeans (Glycine max cv. Buchanan) were studied at one site for three seasons on Cununurra clay in the Ord Irrigation Area. Grain yield increased (P< 0.01) with both primary and residual P in all seasons. When grain yields over the 3 years were standardized to eliminate seasonal effects, relative grain yield (RGY) was positively related to each of bicarbonate-extractable soil P (EP), applied fertilizer P (FP) and fertilizer P applied to the previous crop (PFP). Seventy-three per cent of the variation in relative yield within years could be explained by the equation: RGY = 45.44 + 0.64FP + l.84EP + 0.197PFP R 2 = 0 . 7 3 , P < 0.01 Increasing P status was associated with increased plant height, dry matter production and weight of 100 seeds and increased the number of main-stem nodes, nodules, pods per plant and days to maturity, but had no effect on number of seeds per pod and number of days until commencement and completion of flowering. Concentrations of P in the whole tops and uppermost leaf increased with increasing rate of P application. However, there was a significant P x growth stage interaction on nitrogen concentrations in whole tops and upper-most leaf, these being lower in the high-P plots early in the season. Grain protein content increased with increasing P status while oil content was reduced. The results show that rates of P application higher than those being currently used are required to maximize soybean yield on Cununurra clay.